This invention relates generally to thermal printing, and more particularly to production of multiple thermally printed sheets, such as an "original" and one or more "duplicate" sheets.
Thermal printing has many advantages over impact printing. It is very quiet and has very few moving parts, especially when a thermal linear head is used. Today there are basically three ways to print using thermal head technology The first and oldest method is called "direct thermal printing", the second is thermal transfer, and the third is a variation of thermal transfer.
In direct thermal printing the sheet or paper is coated with a thermal coating containing visually a leuco dye which is exposed when heat is applied by the thermal elements in the thermal head.
Thermal transfer printing employs a plain paper and a thermal ribbon which is coated with a thermo-fusible ink layer containing wax and a colored pigment. The images are transferred to the receiving sheet by the application of heat to the thermal-fusible ink layer. The thermal transfer ribbon can also be made with a sublimable dye layer or an organic-solvent-soluble dye layer and wax.
The variant method uses a leuco dye layer on the image transfer ribbon and a color developer layer on the receiving sheet of paper. In this system the leuco dye when heated reacts with the developer and induces color formation in the leuco dye; and the image is formed on the receiving sheet.
There is need for "two-ply" thermal printing, wherein an "original" and one or more "duplicate" printed sheets are produced, thermally. Prior attempts used a thermo-fusible ink layer containing a wax and colored pigment. This made the top sheet unacceptable because of a black carbon looking coating on the back side. Customer did not accept this, as it would "ink" their hands or clothing.